4 Significant Applications of Vacuum Coating
Vacuum is basically an environment where the gas pressure is less than that of the surroundings. Plasma is a gaseous environment in which there are enough ions and electrons to maintain electrical conductivity. Vacuum technology also referred to as vacuum coating refers to the deposition of a film or a coating in a vacuum environment. Generally, the word vacuum coating is applied to processes that deposit atoms one at a time like the physical vapor deposition (PVD) or low-pressure chemical vapor deposition (LP- CVD PROCESSES. In PVD processes, the material that is deposited comes from the vaporization of a solid or liquid surface. However, in CVD processes, the material being deposited comes from a chemical vapor precursor species that is decomposed by the reduction or thermal decomposition; usually on a hot surface.
In some of the cases the material being deposited reacts with the gaseous environment to form a film of a compound material like an oxide, a nitride, carbide, or a carbo nitride. In CVD processing, the use of plasma to fragment the chemical vapor precursor in the vapor phase enables the decomposition or reduction processes top proceed at lower temperatures than with the thermal activation alone.
Let us now have a look at the applications of vacuum technology used for vacuum coating:
- Electrically Conductive Films
Metal films are some of the most common electrical conductor films. Metal films may be used as blanket metallization’s or can also be formed into discrete conductor lines by masking the substrate during the deposition. Conductor lines are used in the hybrid microcircuit technology and in the manufacture of semiconductor devices. For instance, in semiconductor metallization, aluminum or gold electrode material will diffuse into the silicon during high temperature processing. And an electrically conductive titanium nitride film deposited on the silicon surface before the metal electrode gets deposited that will prevent the diffusion.
- Electrical Insulators
Electrically insulated films are used to electrically isolate the conducting components in semiconductor devices, and as a di-electric within the capacitors. Some of the common insulator film materials are silicon dioxide, aluminum tri-oxide, tantalum pentoxide, silicon nitride, and aluminum nitride. Interposing a thin oxide film between a metal film and a semiconductor enables the formation of the technologically important metal-oxide semiconductor device. Moreover, thick coatings of Silicon di-oxide that has low coefficient of thermal expansion can be deposited.
- Optical Films
Optical films, generally the multilayer films, are the films that affect the optical transmission or reflection of a surface. They are usually alternating layers of materials having high content of Germanium, Silicon, zirconium di-oxide, cerium di-oxide and low indices of refraction. A major application highlights the anti-reflection coatings on the lenses. An example of a broadband filter is an ‘edge filter’ that cuts off the ultraviolet emitted by a mercury vapor lamp. Some of the examples of narrow-band filters are the color filters that are used in photography and in projectors.
- Reflector Coatings
Metal films are widely used for reflector surfaces. Silver metal is often used when corrosion is not a problem, like in the case of back-surface mirrors. Often, the aluminized front-surface reflectors, like the headlight reflectors, are over coated with a protective poly_mer film. Chromium is widely used on front-surface reflectors when corrosion is an issue even though its reflectivity in the visible is far less than that of aluminum. A good example is the cold mirror that reflects the visible radiation but not the infrared wavelengths and heat mirrors that reflect the infrared but not the visible.
Vacuum technology plays a significant role in many coating operations in which the metals, plastics or glass are deposited in the vapor form. A medium or high vacuum is applied to the chamber for its evacuation in the beginning of the process, after which the reaction gases are extracted or the solid raw materials are vaporized.